Proposal
Executive Summary
Water scarcity impacts the American Southwest and is predicted to increase in the coming years stressing an already water-strained region that relies on large volumes of water to produce crops and raise livestock. Membrane desalination allows us to treat nontraditional sources of water (e.g. brackish water, seawater), increasing our available water and alleviating water scarcity. The inevitability of membrane fouling, the deposition of unwanted particles onto a membrane’s surface, raises some challenges that need to be overcome in order to more efficiently treat these nontraditional waters. Fouling increases the energy needed to treat the same volume of water and increases the rate of costly membrane cleanings and replacement. By better understanding fouling we can generate better membrane materials, more robust pretreatment technologies, and more efficient operating parameters. One understudied aspect of fouling is the potential impact of oxyanions on Natural Organic Matter (NOM) fouling in membrane systems. I theorize that oxyanions that interact strongly with calcium will decrease the interactions between cations and the organic matter, decreasing the fouling rate and the concentration of cations in the fouling layer. While some oxyanions that interact weakly calcium but strongly with NOM could accelerate the fouling rate. To test this method we will conduct a series of fouling experiments with divalent cations, oxyanions, and natural organic matter. The impact of the oxyanion will be evaluated based on the surface concentration of the cation on the fouled membrane.
The objective of this research is to study the impact of oxyanions on cation-NOM interactions in the context of ultrafiltration operation. This ICP MS method will allow for the quantification of cations in the fouling layer on the membrane’s surface as well as analysis of cation concentration in the feed solution before and after operation. This method will generate a complete picture of the cation mass balance in the system and will be applied to the analysis of calcium and magnesium.
Discussion of Significance
Calcium and Magnesium are the major divalent cation species found in most natural water systems and both strongly interact with Natural Organic Matter. This method will provide data that in tandem with the performance data (flux decline, permeate rate) gathered during the fouling experiment will provide a holistic understanding of the fate of the cations during this process and the impacts that major oxyanions have on NOM-cation interactions. The feed solution will have cations and anions in the ppm range, but the concentration of our analytes of interest in the digested membrane samples could range from ppb to ppm. This analysis requires high precision and accuracy that spans multiple orders of magnitude. Therefore, solution mode ICP-MS with its high precision and accuracy over a wide concentration range is the best choice for analyzing the metal composition of the fouling layer.
Literature Review
Natural organic matter (NOM) is a complex group of organic molecules formed from decaying plant and animal matter. Primarily composed of fulvic and humic classes of compounds, NOM consists of compounds of varying size and fuctional groups. Divalent cations, particularly calcium, have been shown to have a bridging effect on NOM via the linking of hydrophilic oxygen groups, particularly carboxylic acids (Zhang 2017). This phenomenon can also happen between NOM and hydrophilic membrane surfaces. These interactions can lead to significant fouling and the need for cleaning and eventual replacement of membrane modules. While there has been extensive work on how cations can impact NOM fouling in membrane systems, less work has been done on how oxyanions impact these NOM cation interactions. While this phenomenon has not been studied in a membrane fouling context, some studies have shown that metals can bridge NOM and oxyanions in suspension and at mineral interfaces (Liping 2022, Martin 2017).
The use of ICP-OES/MS to analyze feed solutions and digested/acid leached membrane samples has been used to great effect in previous studies. Some of these studies are “membrane autopsies” where commercial membranes used for utility services are thoroughly analyzed to study the naturally occurring fouling layers (Fortunato 2020, Melian-Martel 2012). Another class of studies are benchtop fouling experiments where researchers control the composition of the feed water and treat this water with either commercial or novel membrane materials until a fouling layer is formed (Landsman 2023, Sarabian 2023). These benchtop studies typically use feed waters that are less complex than natural waters and have much higher concentrations of the analytes of interest to accelerate the fouling process from months to hours. To study the fouling layer these studies dissolve the membrane in concentrated acid solutions, leach the membranes with concentrated acid. While the use of Laser Ablation ICP-MS (LA-ICP-MS) has proven useful in the depth profiling of metal foulants under the membrane’s surface (Wang 2017), this study will focus exclusively on solution mode ICP-MS analysis of feed solutions and dissolved membranes.
This method will look at Calcium and Magnesium. Calcium 40 has a large isobaric interference with Argon 40, but this can be easily avoided by measuring other calcium isotopes. The feed solutions will have complex matrices with natural organic matter, and the dissolved membrane samples will have complex matrices due to leftover organic matter from digestion. Matrix spikes will be important to determine if the analysis will need to pivot from an external calibration method to a standard addition method.
Method and Materials
The membranes will be fouled with a constant flux ultrafiltration system that recirculates the permeate and retentate back into the feed. The feed will consist of Natural Organic Matter, either calcium or magnesium, and either chloride, nitrate, or sulfonate. Samples will be taken from the feed before fouling and after fouling. The part of the membrane closest to the outlet of the system will be removed and its area will be measured. This cut portion of membrane will then be microwave digested in 10 ml of concentrated nitric acid, and then diluted to 100 ml with Mili-Q water. Elemental analysis of Calcium and Magnesium will be run with the ICP MS on the feed samples and the dissolved membrane. An additional dissolved pristine membrane will be analyzed as a method blank. This will lead to the analysis of 19 unknown samples.
This method will follow an external calibration standard method and will use 6 standards for calibration. In order to ensure precision and accuracy a 10x diluted NIST 1643 standard will be used approximately 4 times throughout the run as a quality control check twice after the calibration curve, once before switching from the dissolved samples to the feed samples and at the end of the run. Also, approximately 20 blanks in total will be used before and after the calibration curve and before and after each quality control check.
Magnesium has some minor interferences such as diatomic C12, Calcium has a major isobaric interference with Argon. These interferences can be overcome by fine tuning the torch, running in He mode, and choosing a non interfered Calcium peak for analysis. These samples will have complex matrices due to NOM and the dissolved membrane. In order to assess if the method of standard addition is needed we will perform matrix spikes on one of the dissolved samples and one of the feed samples.
Possible Outcomes
The development and proof of this method will provide a framework for future membrane fouling studies and serve as preliminary data as this study expands to include other common oxyanions such as hypochlorite, phosphate, and arsenate. By better understanding how oxyanions impact the cation NOM interactions, we can identify which oxyanions have a synergistic and which have an antagonistic impact on fouling and develop more selective pretreatment processes to remove certain synergistic oxyanion species. This study could reveal that at these concentrations oxyanions have a minimal impact on cation NOM fouling and pretreatment options can be devised to remove oxyanions to or below these concentrations.
Timeframe and Budget
This method requires running 6 fouling experiments and microwave digesting 6 samples. In terms of time per fouling experiment a 1,000 L fouling experiment with NOM typically takes upwards of 6 hours. The microwave digestion will take 3 hours total; 1 hour for prep, 1 hour to digest, and 1 hour for post-digestion handling. ICP MS analysis for these samples should take close to 2 hours. In total this entire method will take 41 hours to accomplish.
This method proposes analyzing 19 unknown samples, 2 unknown sample spikes (1 for the digested membranes and 1 for the feed solution), 6 calibration standards, a Quality Control Standard 4 times, and upwards of 20 blanks. In total this would require 51 analysis.
At $20 per analysis , this method would require $1,020 for ICP MS analysis. The salts, NOM, and UF system are available for this project in ECJ and are not included in the total monetary costs.